Partition of parathion in synthetic and native membranes

Partition coefficients of [14C]parathion were determined in several types of membranes. Model membranes of egg phosphatidylcholine, dimyristoyl- (DMPC), dipalmitoyl- (DPPC) and distearoyl- (DSPC) phosphatidylcholines, and native membranes of mitochondria, sarcoplasmic reticulum, brain microsomes, myelin and erythrocytes were investigated. Parathion partition is variable among the membranes under study and depends on temperature and cholesterol content. First-order transition of membrane lipids from the gel to the liquid crystalline state is accompanied by a sharp increase in the partition coefficient of parathion. The insecticide is easily accommodated in bilayers of short-chain lipids, since the partitions were 1950, 650 and 270 in DMPC, DPPC and DSPC, respectively, at temperatures 10 degrees C below the midpoint of their transitions. Preferential interaction with short-chain lipids promotes phase separation in heterogeneous bilayers, favouring segregation of lateral domains enriched in insecticide-phospholipid complexes. Cholesterol incorporation in membranes prevents the binding of the insecticide either through competition for similar interacting sites or as a consequence of changes in structural organization of phospholipids.